The two-microelectrode technique of voltage clamping the sheep cardiac Purkinje fiber is being used to examine in detail the time dependence and voltage dependence of contraction in the heart under conditions of rhythmic stimulation and during the transition from rest to thythmic activity. Specific proposals include: an examination of the relation between calcium current and contraction; the determination of the rate and extent of recovery between contractions and how recovery depends on voltage and preceding electrical activity; and a comparison of the dependence of tension on voltage during steady state stimulation with the results of studies performed after rest intervals. The experiments should give insight into the mechanism of excitation contraction coupling in the heart. The data should have a direct bearing on such phenomena as the frequency strength relation, the staircase or treppe phenomenon, and the effects of paired pacing in the heart.